Blue Gas in Tubes - Why? Is it Bad?

H-C, that's funny - I knew the Blue Glow was nothing to worry about for half my life when it came to my guitar amps and their 6L6's. Then recently I changed my stereo rig's amp from one using EL-34's, which never Glowed Blue in that amp, to a pair of mono's using 6550's, which 3/4 of year later are starting to exhibit it. Despite what I always "knew", I had to come and check it out again. That's the difference between guitar players, who'll abuse an amp until it cries uncle and not give a damn before it stops working, and audiophiles, who worry and obssess over every little thing despite everythings' working fine - Even if that guitar player and that audiophile happen to be the same guy! :-)

Elevick and Eldartford, the title of this thread may start with the words "Blue Gas...", but reading the responses and links above should make it clear that this common phenomenon does not usually indicate the presence of undesireable gas inside the tubes. The thread probably should have been less assumptively titled "Blue Glow...". Timwat and Glide3, despite what Kron seems to be inferring, there is really nothing in the KR material to support the contention that typical 'fluorescing'-type Blue Glow is detrimental in any way.

Here is what tube manufacturer Svetlana's website has to say:

Blue Glow -- what causes it?

Glass tubes have visible glow inside them. Most audio types use oxide-coated cathodes, which glow a cheery warm orange color. And thoriated-filament tubes, such as the SV811 and SV572 triodes, show both a white-hot glow from their filaments and (in some amplifiers) a slight orange glow from their plates. All of these are normal effects.

Some newcomers to the tube-audio world have also noticed that some of their tubes emit a bluish-colored glow. There are TWO causes for this glow in audio power tubes - one of them is normal and harmless, the other occurs only in a bad audio tube:

1) Most Svetlana glass power tubes show FLUORESCENCE GLOW. This is a very deep blue color. It can appear wherever the electrons from the cathode can strike a solid object. It is caused by minor impurities, such as cobalt, in the object. The fast-moving electrons strike the impurity molecules, excite them, and produce photons of light of a characteristic color. This is usually observed on the interior of the plate, on the surface of the mica spacers, or on the inside of the glass envelope. THIS GLOW IS HARMLESS. It is normal and does not indicate a tube failure. Enjoy it. Many people feel it improves the appearance of the tube while in operation.

2) Occasionally a tube will develop a small leak. When air gets into the tube, AND when the high plate voltage is applied, the air molecules can ionize. The glow of ionized air is quite different from the fluorescence glow above--ionized air is a strong purple color, almost pink. This color usually appears INSIDE the plate of the tube (though not always). It does not cling to surfaces, like fluorescence, but appears in the spaces BETWEEN elements. A tube showing this glow should be replaced right away, since the gas can cause the plate current to run away and (possibly) damage the amplifier.